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Legumes & Nitrogen Fixation
G2
Nitrogen is essential for life.
Nitrogen is one of the ingredients in proteins, nucleic acids (such as DNA) and other compounds that are
needed by all life forms for growth and development. It is a very abundant element on Earth, but about
93% is stored in rocks in the Earth’s mantle. Most of the remaining 7% is found in the Earth’s atmosphere –
in fact, the air we breathe is approximately 78% nitrogen.
Despite the abundance of nitrogen in the rocks and atmosphere, lack of usable nitrogen in the environment
is one of the key factors that limit plant growth and productivity. Most forms of life are unable to use
nitrogen gas directly from the atmosphere. Plants use nitrogen that has been ‘fixed’ in the form of
nitrogen-containing compounds called nitrate (NO3-) or ammonium (NH4+). Plants can take up nitrate and
ammonium through special transporters on the root surface and then transport the nitrogen into other
parts of the plant, where it is converted into proteins and DNA. Animals obtain all of their nitrogen by
eating plants or by eating other animals.
Legumes and bacteria work together.
Most nitrates are produced by special bacteria through
the process of decomposition of organic matter. Some of
these bacteria, called rhizobia, can occur as free-living
organisms in the soil. However, they also live in symbiotic
relationships with plants in the legume family such as
acacias (known as ‘wattles’ in Australia), beans, peas,
alfalfa and clover. Rhizobia contain an enzyme called
nitrogenase that can convert atmospheric nitrogen into
ammonium that can be used by plants.
In a symbiotic relationship, both partners benefit. In the
case of legumes and rhizobia, the plants receive a steady
source of usable nitrogen from the bacteria, while the
rhizobia get carbohydrates for their energy needs from
the plant.
Rhizobia bacteria (green fluorescent) attached to root hairs of
legume plant.
There are many different species of rhizobia that form symbioses with particular species of legumes based
on specific chemical signal molecules produced by the plant. Plant roots exude chemical compounds called
flavonoids. In some plants, flavonoids are found in flowers, giving them their typical blue and purple
colours. Other flavonoids have health benefits and are found in high levels in soybean products. In legume
roots, flavonoids stimulate rhizobia in the soil to move towards the legume roots and to start making and
colonising a nodule on the root.
Nodules are formed when rhizobia infect the root hairs and form an
infection thread that grows into the inside of the root. The rhizobia multiply
inside the infection thread and a large colony of bacteria builds up inside the
plant. Special cells inside the root (called cortex cells) start to divide in
response to the rhizobia. This is similar to the growth of a tumour, but much
more structured and controlled. Rhizobia move into the new cortex cells and
start the nitrogen fixing symbiosis.
Once a nodule is formed, the bacteria inside it change into a form known as
bacteroids. The bacteroids produce the nitrogenase that converts nitrogen
gas from soil air pockets into ammonium that the plant can use.
Nodules on soybean roots
Nitrogen fertilisers:
Available nitrogen is often a limiting factor for plant growth, so synthetic nitrogen-containing fertilisers
are commonly used in agriculture. However, improper use or overuse of these fertilisers can cause
problems when precipitation carries excess nitrogen into groundwater and rivers, making the water
unsafe to drink.
The production of synthetic nitrogen fertilisers by the Haber-Bosch process is also energy intensive. It
results in an increased use of fossil fuels because it requires extremely high temperatures (in excess of
450°C) and pressures (more than 1,000 times greater than normal atmospheric pressure). In addition, the
fertilisers must be transported to the fields and applied by machine. Its storage also raises security issues
as ammonium nitrate is explosive, especially when mixed with diesel fuel. On a global average, the
production and application of industrial fertilisers is responsible for almost one-third of the energy
needed to produce our food crops.
These problems can be minimised by greater use of legumes as natural fertilisers. In agricultural
techniques such as crop rotation, fields are planted in alternating years with legumes and non-legumes
such as cereal crops. At the end of the growing season, the legumes are ploughed back into the soil where
micro-organisms break down their tissues and convert the organic nitrogen from the plants to inorganic
ammonium and nitrate. This nitrogen is then available for use by the next crop of non-leguminous plants.
CILR research:
Researchers at the ARC Centre of Excellence for Integrative Legume Research are dissecting the genetic
components of the plant and the rhizobia in an attempt to gain a better understanding of these complex
interactions and to develop legumes that can add to the nutrient content of the soil as well as yielding
valuable seed and fodder.
References:
Hirsh, A.M., Lum, M. R. & Downie, J.A., 2001. ‘What makes the rhizobia-legume symbiosis so special?’
Plant Physiology, December 2001, Vol. 127, pp. 1484-1492.
http://academic.reed.edu/biology/Nitrogen/index.html
http://www2.ctahr.hawaii.edu/tpss/research_extension/soliresearch/niftal/nitrogen.html
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